The invention relates to a device for determining the primary current of a current transformer, comprising digital processing means having a first input receiving measured instantaneous values of the secondary current of the transformer and an output supplying the instantaneous values of the associated primary current, corrected in case of saturation of the transformer.
Numerous devices, in particular trip devices of electrical circuit breakers, use current transformers to measure the electrical current in a conductor, either for measuring purposes or for protection purposes. Problems of saturation of the magnetic circuit of a current transformer make it necessary either to overdimension the transformer, which gives rise to problems of volume and cost, or to attempt to correct the effects due to saturation.
A certain number of digital processing methods of the secondary current, which current may be deformed, flowing in a secondary winding of a current transformer have been proposed. The document WO-A-9313581, for example, describes a device enabling corrected primary and secondary currents to be obtained from measured secondary current samples by using at least one saturated current transformer model and one non-saturated current transformer model, and a model of the current primary signals. More recently, the document EP-A-980129 recommended the use of neuron networks performing an inverse transfer function of a current transformer taking the effects of possible saturation of the transformer into account. The neuron network receives on input the secondary current, which may be deformed, flowing in the secondary winding of the current transformer and supplies on output a value representative of the corresponding current to be measured flowing in the primary winding of the transformer.
In practice, known devices only operate correctly if the current to be measured is perfectly sinusoidal. Moreover, as the secondary current is greatly dependent on the load connected to the secondary winding of the transformer, the current transformer can not be used to supply the electronic components of the device.
The object of the invention is to remedy these shortcomings and to provide a device enabling a precise determination of the primary current, even in case of saturation of the transformer, whatever the form of the current, while tolerating large load variations on the transformer secondary.
According to the invention, this object is achieved by the fact that the digital processing means comprise a second input receiving a secondary voltage measured at the terminals of a secondary winding of the current transformer and connected to the input of integrating means supplying signals representative of the instantaneous magnetic flux in the secondary winding of the transformer, modelling means representative of an inverse model of the current transformer and comprising first and second inputs and an output, the first input of the modelling means being connected to the first input of the digital processing means, the second input of the modelling means being connected to the output of the integrating means and the output of the modelling means being connected to the output of the digital processing means.
The modelling means are preferably formed by a numerical table with two inputs. The device according to the invention does not, in this case, require a great computing power, unlike devices of the prior art.
According to a first development of the invention the transformner comprises a first secondary winding supplying the instantaneous values of the secondary current, and a second, open, secondary winding arranged around the first secondary winding and supplying the secondary voltage applied to the second input of the processing means.
According to a second development of the invention, the transformer comprises a secondary winding supplying the instantaneous values of the secondary current and of the secondary voltage, the digital processing means comprising multiplying means connected to the first input of the digital processing means to multiply the secondary current by a quantity representative of the resistance of the secondary winding of the current transformer, summing means connected to the output of the multiplying means and to the second input of the digital processing means, the output of the summing means being connected to the input of the integrating means.